The game Twinbee-RPG seems like a very underrated game and I like to make gameplays for it, but I just can't understand the language of Japanese, otherwise, I don't know what am I selecting, anyone here knows how to translate a PSX game?

That is a very long and complicated process, even much more than what you think buddy.

First of all, the Japanese language is sufficiently complex language when referring to his writing. Their languaje is divided in Katakana, kanji, hiragana (and I'm forgetting something more), if one of that simbols changes, the meaning of the word changes too. It's not so easy like the english, spanish, portuguese or any other language.

And second, you will need an advanced hex editor for hack the PSX inner files, change the meaning of these special symbols and fonts for gameplay, and that is a really hard thing to do.

Is a shame...Konami is company that produces as amazing games like Tokimeki Memorial (also 100% in japanese) or Twinbee RPG, but since they haven't enough popularity these games not reach this part of the continent. Or if we find them, they're 100% in their nature language.

I just can recommend you to find a FAQ of this game, I have finded a FAQ of Tokimemo and with some knowledge of Kanji (that helped of course) I completed that game twice (and this game is hard as hell ).
Better try to familiarize with the japanese language, or find a FAQ, if this doesn't work, try the best method to try-and-fail.

__________________Soldiers... May fortune be with You!!
Multi-Shoot-em-up lover since 1995 and an addict of Tokimemo! Hell-yeah!Now playing: Shin Contra